Abstract

Rice is an important crop that is very sensitive to salinity. However, some varieties differ greatly in this feature, making investigations of salinity tolerance mechanisms possible. The cultivar Pokkali is salinity tolerant and is known to have more extensive hydrophobic barriers in its roots than does IR20, a more sensitive cultivar. These barriers located in the root endodermis and exodermis prevent the direct entry of external fluid into the stele. However, it is known that in the case of rice, these barriers are bypassed by most of the Na(+) that enters the shoot. Exposing plants to a moderate stress of 100 mM NaCl resulted in deposition of additional hydrophobic aliphatic suberin in both cultivars. The present study demonstrated that Pokkali roots have a lower permeability to water (measured using a pressure chamber) than those of IR20. Conditioning plants with 100 mM NaCl effectively reduced Na(+) accumulation in the shoot and improved survival of the plants when they were subsequently subjected to a lethal stress of 200 mM NaCl. The Na(+) accumulated during the conditioning period was rapidly released when the plants were returned to the control medium. It has been suggested that the location of the bypass flow is around young lateral roots, the early development of which disrupts the continuity of the endodermal and exodermal Casparian bands. However, in the present study, the observed increase in lateral root densities during stress in both cultivars did not correlate with bypass flow. Overall the data suggest that in rice roots Na(+) bypass flow is reduced by the deposition of apoplastic barriers, leading to improved plant survival under salt stress.

Results from a typical steady-state experiment for measuring root hydraulic conductivity (Lpr). One-month-old, hydroponically grown rice plants were stressed with 100 mM NaCl for 1 week and allowed to recover in control nutrient solution for 12 h before starting the measurements. (A) Extruded xylem sap in the presence of hydrostatic pressure gradients (Pgas) is plotted against time. (B) Steady-state water flow per unit surface area of the root system (Jvr) as a function of applied driving force. Lpr values were calculated from slopes of the linear ranges of Jvr (P) curves. Lines passing through the data points in B are intended to guide the eye. Circles, Pokkali; triangles, IR20.

Bypass flow of Na+ and PTS in rice roots. One-month-old, hydroponically grown rice plants were stressed with 100 mM NaCl for 1 week and allowed to recover in control nutrient solution for 12 h before starting the measurements. Apoplastic bypass flow is given as the percentage (w/v) of externally applied PTS/Na+. (A) Percentage bypass flow of PTS and Na+ from control and post-stressed (stressed with 100 mM NaCl for 1 week) rice roots with the application of an external gas pressure of 0.1 MPa. (B) Percentage bypass flow of PTS and Na+ from control and post-stressed rice roots with the application of an external gas pressure of 0.2 MPa. (C) Correlation between Na+ bypass flow and amount of suberin in the rice roots. Circles, Pokkali; triangles, IR20. The line passing through the data points in C is intended to guide the eye. Data represent means ±SE, n=10 roots. Asterisks indicate statistically significant differences at *P ≤0.05 and **P ≤0.01 (t-test); the line bars are introduced to indicate the data/bars used for t-test comparison.

Variation in Na+ and K+ uptake by rice cultivars. One-month-old, hydroponically grown rice plants subjected to various salt stress conditions were washed, dried, and acid-digested to estimate Na+ levels by flame photometry. (A) Variation in Na+ uptake by rice shoots under various stress conditions. The bars are labelled with Roman letters I–VI; each letter corresponds to the respective treatment. (B) Change in Na+ amounts in the shoots of rice cultivars with acute (III–I), moderate (IV–II), acute stress with prior moderate stress (VI–V) and Na+ release/efflux upon 1 week of recovery after a conditioning stress (II–V) from A is plotted. (C) Variation in Na+ uptake by rice roots under various stress conditions. (D) Variation in K+ uptake by rice shoots under various stress conditions. (E) Variation in K+ uptake by rice roots under various stress conditions. (F) Na+ amounts in the apoplastic fluid of rice shoots under various stress conditions. Data represent means ±SE, n=6. Asterisks indicate statistically significant differences at *P ≤0.05 and **P ≤0.01 (t-test); the line bars are introduced to indicate the data/bars used for t-test comparison.

Survival of rice plants under salt stress. One-month-old, hydroponically grown rice plants were subjected to various salt stress conditions, and plants that retained more than one-third of their leaf area open without curling up were counted to estimate survival rates. (A) Survival of rice plants under various stress conditions. (B) Correlation between survival and Na+ in the apoplastic fluid in the shoot. (C) Pictures of rice plants under various stress conditions. Data represent means ±SE, n=6 plants. Asterisks indicate statistically significant differences at *P ≤0.05 and **P ≤0.01 (t-test); the line bars are introduced to indicate the data/bars used for t-test comparison. (This figure is available in colour at JXB online.)

Tissue-specific and NaCl-induced transcriptional up-regulation of genes involved in lateral root development in hydroponically grown, 1-month-old Pokkali plants. (A) Tissue-specific expression of RAU1, RAU4, and Arf8 was monitored by RT-PCR in Pokkali plants. L, leaf; St, stem; RB, root basal; RA, root apical. (B) Temporal changes in the RAU1, RAU4, and Arf8 transcript levels were monitored by RT-PCR in Pokkali roots upon stressing with 100 mM NaCl for varying times. ACTIN (Act1) was used as control. In each reaction. 1 μg of total RNA was used. A 20 μl aliquot of each PCR product was loaded onto a 1% agarose gel from a 25 μl reaction.